1. Field of the Invention
The present invention relates to a vehicle wheel. More specifically, the present invention relates to a vehicle wheel having enhanced thermal qualities that result in lower operating temperatures.
2. Description of the Prior Art
In order to brake a vehicle such as an automobile or truck moving horizontally, the kinetic energy of the entire mass of the vehicle must be converted to heat. There is some energy conversion by aerodynamic drag on the vehicle, engine compression, fluid temperature in the transmission and rolling friction of the wheels relative to the road surface, for example. However, the primary mechanism of conversion of this energy to heat is the vehicle brake system. Heat generation rates vary greatly in vehicle brake systems and adequate provision must be made for the dissipation of high heat loads in a short period of time.
When drum brakes are used, the heat is removed from interfacial sliding surfaces. These surfaces may include the brake drum as well as the brake shoes. The brake drum is a fairly massive heat sink, is engaged throughout a relatively large arcuate portion of its internal cylindrical braking surface by the brake shoes, and has the outer surface cooled by convection currents of air. The brake pad shoes have little or no heat sink capability so that most of the heat generated by braking passes directly into the drum. Power input into the drums can heat the internal surface metal of the drum to high temperatures in the 1000.degree. F. range. Much of the heat in the drum is temporarily stored in the surface layer of the metal, and is transferred toward the drum outer surface. The temperature gradient decreases toward the outer surface and heat flows into the wheel(s) and to the air until the drum is cool. This often takes an extended period of time, and drums, wheels and tires may be heated beyond the desired level under severe conditions of repeated heavy braking.
The problem of removal of heat has been attacked by providing heat sinks, air cooling, and liquid cooling. A variety of airflow enhancement devices and designs have been applied to the wheel and brake drum to lower the system operating temperature. British Patent No. 714,997, published Sep. 8, 1954 and U.S. Pat. Nos. 5,320,201; 4,135,764; 2,998,870; and 2,059,170 illustrate the types of devices employed to date in an attempt to lower the temperature of the various brake and wheel system components.
Heat conducted from a vehicle brake drum into the wheel can be a significant factor in the reduction in service life of a tire. Under severe braking events or a sequence of events, the brake system and wheel components can reach elevated temperatures. Heating of the tire can result in chemical changes that cause a breakdown in the structure of the tire. Thus, the temperature of the wheel, especially where the tire contacts the wheel, is important when considering the life expectancy of the tire/wheel/brake system.